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digitalmars.D.learn - case statement allows for runtime values, a case of accepts-invalid?

reply Andrej Mitrovic <none none.none> writes:
int foo(ref int y)
{
    y = 5;
    return y;
}

void main()
{
    int x = 1;
    int y = 2;
    
    switch (x = foo(y))
    {
        case y:
            writeln("x == y");
        default:
    }
    
    assert(x == 5);
    assert(y == 5);
}

According to the docs:
The case expressions must all evaluate to a constant value or array, or a
runtime initialized const or immutable variable of integral type. 

In fact if you try to add a constant, only then will you get an error:
    switch (x)
    {
        case y + 1:
    }
Error: case must be a string or an integral constant, not y + 1

It will also error out if you try to use a field of a struct. Which leads me to
believe the first case should not be allowed to compile. Thoughts?
Apr 19 2011
parent reply Jesse Phillips <jessekphillips+D gmail.com> writes:
Andrej Mitrovic Wrote:


 int foo(ref int y)
 {
     y = 5;
     return y;
 }
 
 void main()
 {
     int x = 1;
     int y = 2;
     
     switch (x = foo(y))
     {
         case y:
             writeln("x == y");
         default:
     }
     
     assert(x == 5);
     assert(y == 5);
 }


Yes bug. Not this part though


     switch (x = foo(y))
Apr 19 2011
next sibling parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
On 4/19/11, Jesse Phillips <jessekphillips+D gmail.com> wrote:

 Yes bug. Not this part though


     switch (x = foo(y))




Yeah that I know.

Do you happen to know if this bug is already filed or should I file it?
Apr 19 2011
parent reply Jesse Phillips <jessekphillips+D gmail.com> writes:
Andrej Mitrovic Wrote:


 On 4/19/11, Jesse Phillips <jessekphillips+D gmail.com> wrote:

 Yes bug. Not this part though


     switch (x = foo(y))




 
 Yeah that I know.
 
 Do you happen to know if this bug is already filed or should I file it?


I would not know. As long as you do a best guess search on what you think it
would be in Bugzilla, post the bug. It is best to have a duplicate here and
there then to miss something. In fact even when something seems similar it is
good to post a new bug when you're not sure (mention possibly related bugs in
the report).
Apr 19 2011
parent reply Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
Got it. Bug is reported.

Btw, is there a specific reason why non-const values are not allowed?

I mean, doesn't a switch statement like this:
switch(value)
{
    case 1:
        foo(); break;
    case 2:
        bar(); break;
    default:
        doo();
}

expand to:

if (value == 1)
    foo();
else if (value == 2)
    bar();
else
    doo();

You can compare anything in an if statement, so why is switch more limited?
Apr 19 2011
next sibling parent Jesse Phillips <jessekphillips+D gmail.com> writes:
Andrej Mitrovic Wrote:


 Got it. Bug is reported.
 
 Btw, is there a specific reason why non-const values are not allowed?
 
 I mean, doesn't a switch statement like this:
 switch(value)
 {
     case 1:
         foo(); break;
     case 2:
         bar(); break;
     default:
         doo();
 }
 
 expand to:
 
 if (value == 1)
     foo();
 else if (value == 2)
     bar();
 else
     doo();
 
 You can compare anything in an if statement, so why is switch more limited?


No, it doesn't lower to an if/else statement. I didn't quite understand the
details, but I'm actually pretty sure it needs a compile-time value.
Apr 19 2011
prev sibling parent bearophile <bearophileHUGS lycos.com> writes:
Andrej Mitrovic:


 Got it. Bug is reported.


Good.



 You can compare anything in an if statement, so why is switch more limited?


switch has stronger requirements than a series of if statements and its uses
such extra information to create assembly code that's more efficient than a
series of if statement, like a dense jump table. (And in some situations there
are ways to produce something even better, if you need to emulate a state
machine or an interpreter. You are able to do it in GCC with computed gotos).

Bye,
bearophile
Apr 19 2011
prev sibling parent Andrej Mitrovic <andrej.mitrovich gmail.com> writes:
*I've searched bugzilla and couldn't find an entry for this particular case.
Apr 19 2011